Method for controlling forces on a strand as it solidifies

ABSTRACT

A method and mechanism for controlling the forces on a partially solidified strand formed in a continuous-casting operation. A speed-regulating tractive force is applied to the strand at a location in the line preceding that where the starter bar is disconnected. In apparatus utilizing a relatively long flexible starter bar, auxiliary tractive forces are applied at preceding locations. In apparatus utilizing a rigid starter bar or a relatively short flexible bar, the auxiliary tractive forces may be applied at locations following the speed-regulating force. The speed-regulating force is maintained at a predetermined maximum, and the auxiliary forces adjusted accordingly to supply the increasing force needed to move the strand, yet avoid excessive tensile or compressive stresses.

ilnite States Patent [191 Gallucci et a1.

11] 3,752,210 [451 Aug. 14, 1973 METHOD FOR CONTROLLING FORCES ON ASTRAND AS IT SOLIDIFIES Filed:

Inventors: Francis Gallucci, lrwin; Frank Slalnar, Monroeville, both ofPa.

Assignee: United States Steel Corporation,

Pittsburgh, Pa.

Aug. 24, 1971 Appl. No.: 174,350

[1.8. CI. 164/82, 164/282 Int. Cl 322d 11/12 Field of Search 164/82,282, 283;

References Cited UNITED STATES PATENTS Rossi 164/282 X Gallucci 164/282Gallucci et a1... 164/282 X Schrewe 164/82 ZillZ-ll Primary Examiner-R.Spencer Annear Attorney-Walter P. Wood [57] ABSTRACT A method andmechanism for controlling the forces on a partially solidified strandformed in a continuouscasting operation. A speed-regulating tractiveforce is applied to the strand at a location in the line preceding thatwhere the starter bar is disconnected. 1n apparatus utilizing arelatively long flexible starter bar, auxiliary tractive forces areapplied at preceding locations. In apparatus utilizing a rigid starterbar or a relatively short flexible bar, the auxiliary tractive forcesmay be applied at locations following the speed-regulating force. Thespeed-regulating force is maintained at a predetermined maximum, and theauxiliary forces adjusted accordingly to supply the increasing forceneeded tomove the strand, yet avoid excessive tensile or compressivestresses.

5 Claims, 5 Drawing Figures Patented Aug. 14, 1973 3 Sheets-Sheet 1Speed No.3 Regulating Drive Drive n v M e m M m e I e! 0 M .s r r m f iI a f o e u a a 0 r n w R M m 0: 4 iv d 8 C, 0 m m m m w m m L m 0 M n Ew. A I v c323 All I! 533583 355k 1 n n D 1 2 N0. 2 Drive 3 Drive l/VVENTORS Meir Attorney Patented Aug. 14, 1973 3 Sheets-Sheet 3 e w r 0 3a N 0. 0. .0 a 0. $6 @Q 0 o 0 0 0 0 we a mvEA/rms FRA/VC/S GALLUCC/ andFRANK sump M Wm their Attorney METHOD FUR CONTROLLING FORCES ON A STRANDAS IT SOLIDIFIES This invention relates to an improved method andmechanism for controlling forces on a continuously cast strandvas itsolidifies.

In a conventional continuous-casting operation, liquid metal is pouredcontinuously into a vertically oscillating, water cooled, open-endedmold, which may be either straight or curved. A partially solidifiedstrand of indefinite length emerges from the bottom of the mold andtravels between series of rolls which engage opposite faces thereof. Inthe example of a straight mold, these rolls may include a straightvertical guide-roll rack immediately beneath the mold, power drivenpinch rolls and/or bending rolls beneath the guide-rollrack, a curvedroll-rack which changes the directionof travel of the strand fromvertical to horizontal, a straightener, optionally in-Iine work-rollstands, andfinally a run-out table on which the strand is severed toappropriate lengths for further processing. Similar parts may be usedwith curved molds, except that there are no straight guide-roll-rack norbending rolls, since the strand is cast with a curvature.

As the strand leaves the mold, only a thin outside skin has solidified,and the core remains liquid. The strand is subjected to intense watersprays as it passes the various rolls, referred to as secondary cooling,whereby it solidifies throughout its cross section by the time it.

is severed. As long as the core is liquid, forces on the strand must becontrolled carefully not to produce defects in the ultimate product.Excessive tensile stresses on a strand may cause it to crack, whileexcessive compression stresses may cause the strand to bulge. An earlierpatent of the present co-inventor Gallucci, US. Pat. No. 3,550,674, andSchrewe US. Pat. No. 3,566,951 describe some of these problems, but themethods and mechanisms heretofore used for limiting such stresses havenot attained precise and positive control we have found desirable.

An object-of our invention is to provide a continuouscasting operationand apparatus in which we employ an improved method and mechanism forcontrolling lengthwise forces on the solidifying strand, therebyavoiding defects in the ultimate product.

A further object is to provide an improved forcecontrolling method andmechanism in which we apply a speed-regulating tractive force to thestrand at a location preceding that where the starter bar isdisconnected (e.g., at the straightener), maintain the speedregulatingforce at a predetermined maximum, and apply controlled auxiliarytractive forces at other locations to minimize or eliminate stresses inthe strand.

A further object, applicable to apparatus utilizing a relatively longflexible starter bar, is to provide a force controlling method andmechanism in which we subject the strand to a speed-regulating drive atan advanced location in the line, and to a series of adjustableauxiliary tractive forces along its length between the mold and thespeed-regulating drive, whereby we maintain lengthwise forces in thestrand at any desired value or eliminate tension altogether in favor ofa compression stress.

In the drawing:

FIG. 1 is a partly diagrammatic vertical section of a continuous-castingapparatus which utilizes a relatively long flexible starter bar and isequipped with one embodiment of our force-controlling mechanism, the

2 parts being shown in the position they occupy shortly after thestartof a casting operation;

FIG. 2 is a graph showing a typical relation of the forces on a strandin an apparatus such as that shown in FIG. 1;

FIG. 3 is a block diagram of an electric circuit suitable for use in theembodiment of our mechanism shown in -FIG. l;

FIG.4 is a block diagram of a similar circuit, but embodying computercontrol for automatic operationjand FIG. 5 is a partly diagrammaticvertical section of a continuous-casting apparatus which utilizes arigid starter :bar and is equipped with a modified embodiment of ourforce-controlling mechanism, the parts :being shown in the position theyoccupy after the casting operation is underway and the starter bardisconnected.

The continuous-casting apparatus illustrated in FIG. 1 includes from topto bottom a straight mold 10, a

straight vertical guide-roll-rack 12, a bending roll unit 13, a curvedroll-rack 14, a straightener l5, and a runout table 16. Liquid metal isintroduced to the mold .from a tundish 17 supported thereabove. At thebeginning of -a casting operation, a starter bar 18 is inserted in thelower end of the mold and extends through and beyond the straightener15. A partially solidified strand .8 emerges from the lower end of themold and descends following the starter bar between the various sets ofrolls. After the leading end of th strand S clears the straightener 15,the starter bar is disconnected and stored in readiness to start thenext cast.

Numerous variations in the structure of the casting apparatus of courseare possible. The apparatus illustrated in FIG. I utilizes a relativelylong flexible starter bar, which is not disconnected until it reachesthe run- .out table, but our invention in modified form can be used withapparatus which utilizes a starter bar (rigid .or flexible) disconnectedabove the bending rolls, as shown in FIG. 5 and hereinafter described.Also it can be used with apparatus which has a curved mold, or which hasa varying radius of curvature in the curved roll-rack, or which hasin-line work rolls, etc., all of which are well known in thecontinuous-casting art.

In accordance with the embodiment of our invention illustrated in FIG.1, the apparatus has sets of driven rolls 20 and 20a which engageopposite faces of first the starter bar 18 and later the strand S at anadvanced location in the line and which we refer to as ourspeed-regulating drive." This embodiment has a first set of auxiliarydriven rolls 21, which engage opposite faces of the strand near thebottom of the mold and which we refer to as our No. 1 drive. Between ourNo. 1 drive and out speed-regulating drive, the apparatus haslongitudinally spaced sets of auxiliary driven rolls 22 and 23, which werefer to as our "No. 2 drive and No. 3 drive" respectively. All theother rolls shown can be idlers. In the apparatus illustrated, the rolls20 and 20a of our speed-regulating drive are lo cated fore and aft ofthe straightener 15; our No. 1 drive 21 is located between theguide-roll rack 12 and the bending roll 13; our No. 2 and 3 drives 22and 23 are located intermediate the length of the curved rollrack 14.Nevertheless it is apparent that the exact location and number of thesedrives can vary.

At our No. 1 drive 21 the strand S has only a thin skin which is easilyruptured. Hence the pressure which each roll of this drive exertsagainst the strand must be relatively low. In order to produce thenecessary tractive force on the strand, as hereinafter explained,without exerting excessive pressure at any one location thereon, we usea plurality of closely adjacent pairs of rolls at this drive (four pairsin the illustration). At our No. 2 and 3 drives 22 and 23, the strandstill has a liquid core, but its skin is thicker. For the same reason,we prefer to use a plurality of pairs of rolls also at each of thesedrives, but the number can be smaller (two pairs each in theillustration). Likewise we use two pairs of rolls in thespeed-regulating drive, but place them on opposite sides of thestraightener, as already described.

At the beginning of a casting operation, when the starter bar 18 extendsbetween the various sets of rolls, our speed-regulating drive 20, 20aacts as a brake to restrain the starter bar from descending under itsown weight. Initially we operate our auxiliary drives with their currentset points at zero, whereby they act as idlers. As the casting operationproceeds, the leading end of the strand S passes through the guideroll-rack 12, our No. 1 drive 21 (operating as an idler at this time),and into the bending roll unit 13. As soon as the strand commences tobend, some tractive force is needed to propel it. At first thespeed-regulating drive 20, 20a furnishes this force. The farther thestrand progresses, the greater the resistance to its movement and thegreater the tractive force or torque needed to propel it at a givenspeed.

Conveniently the motors for the various drives can be constant-field d-cmotors, which have a characteristic that at any speed their torqueoutput is directly proportional to the current valve. When the curent tothe speed-regulating drive reaches or commences to exceed apredetermined maximum (for example amperes), we increase the current toour No. 1 drive to supply the increasing tractive force needed to propelthe strand, and thereafter we maintain the current to thespeed-regulating drive at this maximum. When the leading end of thestrand successively passes through our No. 2 and 3 drives 22 and 23, weincrease the current to each of these drives in turn to supply theincreasing tractive force, thereafter keeping the current constant tothe speed-regulating drive and to No. 1 drive, and subsequently to No. 2drive after No. 3 drive takes over. After the casting operation is fullyunderway and the strand extends all the way from the mold 10 to therun-out table 16, the current to the speedregulating drive 20, 20a mayremain at the exemplary maximum of 10 amperes, and the current to theNo. l, 2 and 3 drives 21, 22 and 23 may be at 20, and 10 amperesrespectively.

FIG. 2 is a graph illustrating a typical force profile on the strandwhen our method is practiced with the casting apparatus shown in FIG. 1.The abscissae represent distances from the bottom of mold 10. Positiveordinates represent tensil forces on the strand and negative ordinatescompressive forces. Curve A represents the line resistance to movementof the strand through the casting apparatus. Curve B represents theforce tending to move the strand through the apparatus under its ownweight. Curve C represents the difference between Curves A and B, or thetractive force which must be supplied to move' the strand. Curve C alsorepresents the magnitude of tensil stresses in the strand at variouspoints along its length if the speed-regulating drive alone were used topull the strand through the apparatus. Curve D shows an example of themagnitude of tensil stresses in the strand when our method is followed.From the mold 10 to our No. 1 drive, Curve D is the same as Curve C. AtNo. 1 drive we apply a tractive force to the strand and thereby place itin compression. Following No. 1 drive Curve D rises parallel with CurveC until it reaches No. 2 drive, where we again apply a tractive forcewhich restores the compression. The same relation takes place betweenNo. 2 drive and No. 3 drive, and again between No. 3 drive and thespeed-regulating drive. In this manner the strand remains under slightcompression substantially throughout the operation.

FIG. 3 illustrates in block diagram a typical electric circuit we canuse for manually controlling the various drive motors. Constant-fieldd-c motors for the two sets of rolls 20 and 20a of the speed-regulatingdrive are indicated at 26 and 26a respectively. We energize these motorsthrough a closed-loop speed-regulating circuit 28, which includes apower supply 29, an amplifier 30 and a tachometer 31. We control thespeed of motors 26 and 260 through a master pilot 32, which we canadjust either manually or through a liquid-level control mechanism onthe mold, as shown for example in Tiskus et al. US. Pat. No. 3,300,820.The two motors 26 and 26a run at essentially a constant speed, which wechange only to change the strand speed. An ammeter 33 indicates themagnitude of current the motors draw to maintain this speed, whichmagnitude is a measure of the torque output of the motors.

When a casting operation is getting underway, the magnitude of currentwhich the motors 26 and 26a draw continually increases, since the lineresistance to movement of the strand increases, as shown by Curves C andD of FIG. 2. When this current reaches a predetermined magnitude (forexample 10 amperes) as indicated by ammeter 33, we increase the currentto our No. I drive 21, which has a constant-field d-c motor 36 energizedthrough a closed-loop current-regulating circuit 37. The latter includesa power supply 38, an amplifier 39, a manual load adjustment 40 and anammeter 41. We connect the master pilot 32 to the amplifier 39, wherebymotor 36 rotates the rolls of No. 1 drive at the same peripheral speedas the rolls of the speedregulating drive 20, 200. We gradually increasethe current to motor 36 as needed to maintain the current to motors 26,and 26a at the predetermined magnitude as a maximum. Thus No. 1 drivesupplies the increasing tractive force needed to move the strand. Atthis time the leading end of the strand. S has passed No. 1 drive and isapproaching No. 2 drive 22.

Our-No. 2 snd 3 drives 22 and 23 have closed-loop current-regulatingcircuits 44 and 45 which are similar to circuit 37 of No. 1 drive. Afterthe leading end of the strand reaches No. 2 drive, we maintain thecurrent to both motors. 26 and 26a of the speed-regulating drive and tomotor 36 of our No. 1 drive at constant magnitudes and increase-thecurrent to No. 2 drive to supply the increasing tractive force needed tomove the strand. After the leading end reaches No. 3 drive, we use thisdrive in the same fashion, keeping the current to No. 2 drive constant.

FIG. 4 illustrates in block diagram the way in which we can make theforegoing adjustments automatically. We connect a pulse generator 46 tomotor 26:; of our speed-regulating drive to track the leading end of thestrand. We connect the pulse generator 46 to a digital computer 47,which we in turn connect to the control circuits 37, 44 and 45 for No.1, 2 and 3 drives. In this manner the computer automatically adjusts thecurrent to each drive to achieve the same effect as in the manualcontrol already described.

FIG. 5 illustrates a continuous-casting apparatus in which the starterbar 50 is rigid and to which we may apply our invention in modifiedform. The apparatus includes from top to bottom a straight mold 51, astraight vertical guide-roll-rack 52, power-driven pinch rolls 53, acurved roll-rack 54, a straightener 55, and a horizontal roll-rack 56.The curved roll-rack has at least one switch section 57 in which some ofthe rolls are journaled. The switch section can open to allow thestarter bar 50 to decend vertically after it is disconnected from theleading end of the strand, as shown for example in Foldessy US. Pat. No.3,338,297.

When we apply our invention to an apparatus constructed as shown in FIG.5, the pinch rolls 53 become the speed-regulating drive. We use aplurality of closely adjacent pairs of pinch rolls to spread the forcealong the strand, similar to the No. 1 drive of FIG. 1. At the beginningof a casting operation, the pinch rolls 53 engage the starter bar andact as a brake to restrain it from descending under its own weight.After the starter bar is disconnected and the switch section 57 closed,the strand S enters the curved roll-rack 54 and requires an increasingtractive force to propel it. At spaced locations along the curvedroll-rack, .we provide auxiliary pairs of driven rolls 58, 59 and 60which we again refer to as our No. l, 2 and 3 drives. The rolls in thestraightener 55 are driven, but all the other rolls can be idlers.

At the beginning we operate the auxiliary drives 58, 59 and 60 asidlers, the same as in the apparatus shown in FIG. 1. When the leadingend of the strand passes through No. 1 drive 58, we increase the currentto this drive to supply the increasing tractive force needed to propelthe strand, and thereafter maintain the current to the speed-regulatingdrive (pinch rolls 53) constant.

When the leading end of the strand successively passesthrough our No. 2and 3 drives 59 and 60 and finally the straightener 55, we increase thecurrent to each, the same as in FIG. 1, keeping the current to thepreceding drives constant.

From the foregoing description, it is seen that our invention assuresthat a solidifying continuously-cast strand is relieved of any excessivelongitudinal stresses as it solidifies. Preferably we maintain a smallcompressive stress during most of the operation, as Curve D of FIG. 2indicates. In the apparatus shown in FIG. l, the speed-regulating drivepulls the strand, and our auxiliary drives act to relieve excessivecompressive stresses which tend to bulge the strand, as well asexcessive tensile stresses. In this manner our invention avoids defectsin the strand caused by lengthwise stresses, and permits a highercasting speed for crack-sensitive steel grades.

We claim:

1. In a continuous-casting operation in which a partially solidifiedstrand of indefinite length descends from the bottom of a mold andtravels between series of rolls which engage opposite faces thereof;

at least some of said rolls defining a curved path in which the strandbends, whereby its direction of travel becomes substantially horizontal;and

a starter bar is connected to the leading end of said strand as theleading end emerges from the mold, but is disconnected therefrom at alocation spaced below the mold;

an improved method of controlling forces on said strand comprising:

initially applying, through selected rolls of said series at a locationpreceding the location at which the starter bar is disconnected, a forcerestraining descent of said starter bar and said strand;

subsequently using said selected rolls to apply a speedregulatingtractive force to said strand as the leading end thereof advancesthrough said curved path and a tractive force is required to propel thestrand;

initially operating other selected rolls of said series substantially asidlers, which other selected rolls have the capability of being drivenand of applying auxiliary tractive forces to said strand; energizing thedrive to the first of said other selected rolls nearest the mold whensaid speed-regulating tractive force reaches a predetermined maximum;

sequentially energizing the drives to the succeeding other selectedrolls as the leading end of said strand advances farther along saidcurved path and the force on the immediately preceding other selectedrolls in turn reaches a predetermined maximum;

whereby lengthwise stresses in said strand are minimized through theoperation.

2. A method as defined in claim I in which the speedregulating force isapplied to the strand at an advanced location in the line and pulls thestrand through the line, and said auxiliary forces are applied atlocations preceding said speed-regulating force and act to relievetensile stresses in the strand.

3. A method as defined in claim 1 in which the line includes astraightener following said curved path, said speed-regulating forcebeing applied at said straightener.

4. A method as defined in claim 3 in which the first of said auxiliaryforces is applied at a location near said mold, and the others ofauxiliary forces are applied at locations between the first auxiliaryforce and said speed-regulating force.

5. A method as defined in claim 1 in which the speedregulating force isapplied to the strand at a location near the mold and pushes the strandthrough the line, and said auxiliary forces are applied at locationsfollowing said speed-regulating force and act to relieve exces-' sivecompressive stresses in the strand.

' a a: 1: a a

1. In a continuous-casting operation in which a partially solidifiedstrand of indefinite length descends from the bottom of a mold andtravels between series of rolls which engage opposite faces thereof; atleast some of said rolls defining a curved path in which the strandbends, whereby its direction of travel becomes substantially horizontal;and a starter bar is connected to the leading end of said strand as theleading end emerges from the mold, but is disconnected therefrom at alocation spaced below the mold; an improved method of controlling forceson said strand comprising: initially applying, through selected rolls ofsaid series at a location preceding the location at which the starterbar is disconnected, a force restraining descent of said starter bar andsaid strand; subsequently using said selected rolls to apply aspeedregulating tractive force to said strand as the leading end thereofadvances through said curved path and a tractive force is required topropel the strand; initially operating other selected rolls of saidseries substantially as idlers, which other selected rolls have thecapability of being driven and of applying auxiliary tractive forces tosaid strand; energizing the drive to the first of said other selectedrolls nearest the mold when said speed-regulating tractive force reachesa predetermined maXimum; sequentially energizing the drives to thesucceeding other selected rolls as the leading end of said strandadvances farther along said curved path and the force on the immediatelypreceding other selected rolls in turn reaches a predetermined maximum;whereby lengthwise stresses in said strand are minimized through theoperation.
 2. A method as defined in claim 1 in which thespeed-regulating force is applied to the strand at an advanced locationin the line and pulls the strand through the line, and said auxiliaryforces are applied at locations preceding said speed-regulating forceand act to relieve tensile stresses in the strand.
 3. A method asdefined in claim 1 in which the line includes a straightener followingsaid curved path, said speed-regulating force being applied at saidstraightener.
 4. A method as defined in claim 3 in which the first ofsaid auxiliary forces is applied at a location near said mold, and theothers of auxiliary forces are applied at locations between the firstauxiliary force and said speed-regulating force.
 5. A method as definedin claim 1 in which the speed-regulating force is applied to the strandat a location near the mold and pushes the strand through the line, andsaid auxiliary forces are applied at locations following saidspeed-regulating force and act to relieve excessive compressive stressesin the strand.